1. Field of the Invention
This invention relates to a method for the production of acrolein and acrylic acid. More particularly, in the production of acrolein and acrylic acid by the catalytic gas phase oxidation of propylene with a molecular oxygen-containing gas, the present invention relates to a method for enabling acrolein and acrylic acid to be stably produced with a high yield for a long time while effectively repressing the formation of such by-products as various aldehydes, carboxylic acids, high boiling compounds, and tarry substances without impairing the performance of a catalyst.
2. Description of the Related Art
The practice of producing acrolein and acrylic acid by the catalytic gas phase oxidation of propylene with a molecular oxygen-containing gas is widely prevailing on a commercial scale. Generally, the production of acrylic acid is effected by a two-step reaction which comprises a former step of producing acrolein mainly by the gas phase oxidation of propylene with a molecular oxygen-containing gas and a latter step of further oxidizing the reaction gas containing the acrolein thereby producing acrylic acid.
At the former step for oxidizing propylene with a molecular oxygen-containing gas, however, a side reaction occurs to produce organic acids such as terephthalic acid and maleic acid, high boiling compounds, and tarry compounds besides the main reaction which produces acrolein. As a result, these by-products contaminate the reaction apparatus and, in an extreme case, prevent the reaction apparatus from being normally operated because they form deposits such as carbonized materials, aggravate pressure drop, and plug pipes or degrades the quality of product. If the reaction apparatus is provided with purification means for the removal of these by-products, the provision will entail such problems as increasing the cost of equipment, boosting the cost of production. Further, acrolein and other highly reactive compounds give rise to after reaction.
Heretofore, for the purpose of repressing these side reactions, such measures as suddenly cooling the outlet gas of a reaction vessel of the former step thereby repressing the formation of side reactions or retaining the temperature of the outlet gas of the reactor above a fixed level thereby repressing the contamination of a reaction apparatus due to the deposition of high boiling compounds and tarry compounds have been resorted to (as disclosed in U.S. Pat. No. 3,876,693, for example).
These measures, however, necessitate special devices adapted exclusively for their own purposes and entail consumption of a large amount of energy for cooling or heating and consequently prove economically disadvantageous because of an inevitable addition to the cost of production. Besides, they are deficient in effect. Especially, they are not ratable as fully effective regarding the contamination of the apparatus with organic acids, high boiling compounds, and tarry compounds, the deposition of carbonized materials, and the deterioration of the quality of product.
As concerns the reclamation of a reaction product gas as part of a material gas, a method for limiting the amount of organic compounds such as organic acids has been proposed (as disclosed in U.S. Pat. No. 4,147,885, for example). This patent application, however, has absolutely no mention of the quality of the raw material propylene gas.
It is an object of this invention, therefore, to provide in the production of acrolein and acrylic acid by the catalytic gas phase oxidation of propylene with a molecular oxygen-containing gas, a method which enables acrolein and acrylic acid to be stably produced with a high yield for a long time from propylene by effectively repressing such side reactions as are responsible for the formation of organic acids, high boiling compounds, and tarry compounds, the deposition of carbonized materials, and the deterioration of the quality of products.
This invention, in a process of two-step reaction for the production of acrylic acid by the catalytic gas phase oxidation of propylene with a molecular oxygen-containing gas, is directed at providing a method for stably and continuously for a long time producing acrolein mainly from propylene with a high yield and consequently producing acrylic acid ultimately with a high yield by effectively repressing such side reactions as are responsible for the formation of organic acids, high boiling compounds, and tarry compounds, the deposition of carbonized materials, and the deterioration of the quality of products.
We have performed a study with a view to solving the problems encountered in the production of acrolein and acrylic acid by the catalytic gas phase oxidation of propylene with a molecular oxygen-containing gas as described above and, as a result, have acquired the following knowledge.
(1) The organic acids, high boiling compounds, and tarry compounds are formed by the side reactions irrespectively of differences in reaction conditions, activity of catalyst, or selectivity of the formation of acrolein.
(2) While these by-products occur only in extremely small amounts as compared with acrolein and acrylic acid which are main products, they form a serious hindrance to a continuous and stable operation.
(3) The amounts of their formation are affected more by the impurities contained in the raw material propylene and their amounts than by the performance of a catalyst.
(4) The presence of unsaturated compounds other than propylene largely affects the performance of a catalyst and the amounts of by-products to be formed.
(5) The effects on the performance of a catalyst and the amounts of by-products to be formed increase in accordance as the degree of unsaturation of impurities in propylene is heightened or the basicity gains in intensity.
Then, we have continued the study and have discovered that the side reactions are effectively repressed and the problems mentioned above are consequently solved by lowering the total content of unsaturated hydrocarbons of 2-5 carbon atoms other than propylene in the raw material propylene below 500 ppm (by weight). The present invention has been perfected based on this knowledge.
Specifically, in the production of acrolein and acrylic acid by the catalytic gas phase oxidation of propylene with a molecular oxygen-containing gas in the presence of an oxidation catalyst, this invention concerns a method for the production of acrolein and acrylic acid which comprises lowering the total content of unsaturated hydrocarbons of 2-5 carbon atoms (excluding propylene) in the raw material propylene below 500 ppm (by weight).
Further in the production of acrylic acid by a two-step reaction comprising a former step for producing acrolein mainly by the catalytic gas phase oxidation of propylene with a molecular oxygen-containing gas in the presence of an oxidation catalyst and a latter step for producing acrylic acid by the gas phase oxidation of the acrolein-containing reaction gas in the presence of an oxidation catalyst, this invention concerns a method for the production of acrylic acid which comprises lowering the total content of unsaturated hydrocarbons of 2-5 carbon atoms (excluding propylene) in the raw material propylene below 500 ppm (by weight).
The characteristic feature of this invention comprises using a raw material propylene of which the total content of unsaturated hydrocarbons of 2-5 carbon atoms except that propylene (hereinafter referred to simply as xe2x80x9cunsaturated hydrocarbons of 2-5 carbon atomsxe2x80x9d) is below 500 ppm (ppm by weight; applicable similarly hereinafter). The content of the unsaturated hydrocarbons of 2-5 carbon atoms is preferred to be not more than 450 ppm, more preferred to be not more than 300 ppm, and particularly preferred to be not more than 200 ppm.
In this invention, propylene having a content of unsaturated hydrocarbons of 2-5 carbon atoms of not more than 500 ppm, preferably not more than 450 ppm, more preferably not more than 300 ppm, and particularly preferably not more than 200 ppm and also having a total content of diene and acetylenic compounds of 2-5 carbon atoms of not more than 200 ppm, preferably not more than 150 ppm, more preferably not more than 100 ppm, and particularly preferably not more than 50 ppm is used advantageously. As one ideal example of the raw material propylene, propylene having a content of unsaturated hydrocarbons of 2-5 carbon atoms of not more than 200 ppm and a total content of diene and acetylenic compounds of 2-5 carbon atoms of not more than 100 ppm, particularly preferably not more than 50 ppm may be cited.
Butadiene may be cited as a typical example of the diene and methyl acetylene as a typical example of the acetylenic compounds, both mentioned above. As other typical examples of the unsaturated hydrocarbons of 2-5 carbon atoms, ethylene, butylene, and isobutylene may be cited.
It is such saturated hydrocarbons as propane that account for the largest proportion of the impurities in the raw material propylene. No particular limit is imposed on the content of such saturated hydrocarbons because these saturated hydrocarbons have low reactivity with a catalyst and exert only small influence on the performance and service life of the catalyst. In contrast, unsaturated hydrocarbons have high reactivity with a catalyst, induce side reactions, and by-produced substances other than the compounds aimed at. In addition to lowering the catalytic activity and decreasing the yields of acrolein and acrylic acid, the unsaturated hydrocarbons form a cause for impairing the service life of the catalyst. Further, the by-products form one cause for the generation of after reactions, contaminate a reaction apparatus, and cause plugging of pipes. Thus, the concentration of unsaturated hydrocarbons in the raw material propylene must be strictly controlled.
The raw material propylene contains involatile residues and sulfur components in addition to the saturated and unsaturated hydrocarbons mentioned above. These impurities form a cause for degradation of the capacity of a propylene evaporator and a cause for corrosion of the reaction apparatus. It is, therefore, proper to keep the contents of involatile residues and sulfur components each below 100 ppm (by weight), preferably below 50 ppm (by weight).
The raw material propylene usable herein does not impose any restriction particularly but only requires to have the content of unsaturated hydrocarbons within the range mentioned above. For example, the species of propylene produced by the method of steam cracking of naphtha and the species of propylene produced by the method of dehydrogenation or oxidative dehydrogenation of propane can be used as the raw material propylene. The method for adjusting the content of unsaturated hydrocarbons in such propylene does not impose any restriction particularly. In the case of the method of steam cracking naphtha, for example, this adjustment may be accomplished by separating the unsaturated hydrocarbons by the technique of low-temperature fractional distillation and then removing acetylenic compounds entrained by the unsaturated hydrocarbons in a minute amount by selective hydrogenation. The propylene may be otherwise purified by super precision distillation using an increased number of steps.
No particular limit is imposed on the purity of propylene in the raw material propylene. In consideration of the economy to be associated with the purification, the raw material propylene having a purity of propylene of not less than 90%, preferably not less than 92%, is advantageously used.
Incidentally, the content of unsaturated hydrocarbons of 2-5 carbon atoms in the raw material propylene is preferred to be as small as possible. If this content is lowered to below 1 ppm, however, a further improvement in yield enough to offset the increase in cost due to the extra work of purification will not be recognized. It is enough for the contents of diene and acetylenic compounds of 2-5 carbon atoms to be lowered to the neighborhood of 0.1 ppm.
The production of acrolein and acrylic acid from propylene and the production of acrylic acid by a two-step reaction from propylene which are contemplated by this invention can be carried out by a method which is generally used or which is known to be used except that the point that the raw material propylene to be used ought to have a content of unsaturated hydrocarbons of 2-5 carbon atoms of not more than 500 ppm.
As respects the catalyst for the production, for example, a catalyst which is generally used for the production of acrolein and acrylic acid from propylene and the production of acrylic acid by a two-step reaction from propylene can be used. In the production of acrylic acid by the two-step reaction, for example, a metal oxide catalyst having molybdenum and bismuth as essential components may be used as the oxidation catalyst in the former step. Among other metal oxide catalysts conceivable herein, oxidation catalysts represented by the general formula (1) may be cited as preferable examples.
MoaBibFecAdBeCfDgOxxe2x80x83xe2x80x83(1)
In the formula, Mo is molybdenum, Bi is bismuth, Fe is iron, A is at least one element selected from the group consisting of cobalt and nickel, B is at least one element selected from the group consisting of alkali metals, alkaline earth metals, and thallium, C is at least one element selected from the group consisting of tungsten, silicon, aluminum, zirconium, and titanium, D is at least one element selected from the group consisting of phosphorus, tellurium, antimony, tin, cerium, lead, niobium, manganese, arsenic, and zinc, O is oxygen, and a, b, c, d, e, f, g, and x is the atomic ratios respectively of Mo, Bi, Fe, A, B, C, D, and O such that b=0.1-10, c=0.1-20, d=2-20, e=0.001-10, f=0-30, g=0-4, and x is a numerical value to be determined by the state of oxidation of the component elements.
The oxidation catalyst may be in the shape of cylinders, rings, or spheres or may be in an amorphous form. Optionally, it may be supported on an inert carrier or it may be obtained by molding an active component by a suitable method. Besides the components mentioned above, the oxidation catalyst may incorporate additionally therein a molding additives and a reinforcing agent. For example, various glass fibers and whiskers may be used for the incorporation.
The oxidation catalyst does not need to be limited to a simple species. It is allowed to have part thereof diluted with an inert carrier. Alternatively, several species of oxidation catalysts of varying activity prepared by altering components, method of manufacture, and conditions of calcination may be used in a combined form.
As the reactor, a shell-and-tube type fixed bed reactor is generally adopted. It is also allowable to use a fluidized bed type reactor or a moving bed type reactor. The reactor is not particularly discriminated on account of the kind of material used therefor. It may be made of carbon steel or stainless steel, for example.
The molecular oxygen-containing gas source is not limited to pure oxygen. Air and waste gases emanating from various plants may be suitably used. Other impurities than the unsaturated hydrocarbons mentioned above, namely carbon dioxide gas and other impurities such as NOx, SOx, and moisture which occur in such amounts as found in the standard air bring no particular effect.
The unreacted propylene which remains after such soluble components as acrylic acid have been recovered and separated by the use of a solvent formed substantially of water or other solvent from the acrylic acid-containing reaction gas obtained at the latter step in the production of acrylic acid by the catalytic gas phase oxidation such as, for example, a two-step reaction, of propylene can be circulated and used wholly or partly as the raw material gas. In this case, the raw material propylene which is composed of the propylene so circulated and the propylene to be freshly supplied is only required to have a content of unsaturated hydrocarbons of 2-5 carbon atoms of not more than 500 ppm, preferably not more than 450 ppm, more preferably not more than 300 ppm, and particularly preferably not more than 200 ppm and a total content of diene and acetylenic compounds of 2-5 carbon atoms of not more than 200 ppm, preferably not more than 150 ppm, more preferably not more than 200 ppm, and particularly preferably not more than 50 ppm.
The method of this invention is executed by first packing a shell-and-tube type reactor with the oxidation catalyst mentioned above and passing a raw material gas composed of raw material propylene, a molecular oxygen-containing gas, and an optionally incorporated inert gas through the reaction zone of the reactor at a reaction temperature in the range of 250-450xc2x0 C., preferably in the range of 280xc2x0-400xc2x0 C., at a space velocity in the range of 300-5,000 hrxe2x88x921, preferably in the range of 700-3,000 hrxe2x88x921.
Since this invention effectively represses side reactions, it can solve such problems as by-products of organic acids, high boiling compounds, and tarry compounds, deposition of carbonized materials, consequent aggravation of pressure drop, plugging of tubes, and deterioration of quality of product.
According to this invention, the deterioration of the performance of the oxidation catalyst to be used can be repressed effectively. By this invention, therefore, acrolein and acrylic acid can be stably produced continuously for a long time with a high yield from propylene. Particularly in the case of producing acrylic acid by the two-step reaction, acrolein is mainly produced from propylene with a high yield at the former step and consequently acrylic acid is produced with a high yield at the latter step, stably for a long time.